1. Field of Invention
The present invention relates to a high electron mobility transistor (HEMT) and a manufacturing method thereof; particularly, it relates to an enhanced mode HEMT and manufacturing method thereof.
2. Description of Related Art
FIGS. 1A and 1B show a schematic cross-section view and a band diagram of a prior art high electron mobility transistor (HEMT) 100. As shown in FIG. 1A, a gallium nitride (GaN) layer 12 is formed on a substrate 11, and an isolation region 13 is formed in the GaN layer 12. The isolation region 13 for example is formed by a shallow trench isolation (STI) process or a local oxidation of silicon (LOCOS) process, the former being shown in the figure. The HEMT 100 further includes an aluminum gallium nitride (AlGaN) layer 14, a gate 15, a source 16, and a drain 17 besides the GaN layer 12 and the isolation region 13. A two dimensional electron gas (2DEG) 18 is formed at the junction between the GaN layer 12 and the AlGaN layer 14, and the 2DEG 18 is electrically connected to the source 16 and the drain 17. As shown in FIG. 1B, The Fermi level Efs of the GaN layer 12 and the Fermi level Efb of the AlGaN layer 14 are at the same level. The conduction levels, i.e., the lowest level of the conduction band, Ecs of the GaN layer 12 and Ecb of the AlGaN layer 14, and the valence levels, i.e., the highest level of the valence band, Evs of the GaN layer 12 and Evb of the AlGaN layer 14, are bended at the junction of the GaN layer 12 and the AlGaN layer 14, such that the electrons are trapped in the electron well 18a. These trapped electrons can eliminate Coulomb scattering to increase the electron mobility in the 2DEG 18, such that the operation speed of the HEMT 100 is faster than a conventional semiconductor device at ON state.
However, the HEMT 100 is a depletion device, i.e., the gate voltage of the HEMT 100 is negative during normal operations. In practical applications, it is not convenient to adopt and operate a depletion device, especially in high frequency applications. A positive gate voltage of an HEMT during normal operations can decrease the complexity of the circuitry and the manufacturing cost.
In view of above, to overcome the drawbacks in the prior art, the present invention proposes an enhanced mode HEMT and a manufacturing method thereof which provide a lower manufacturing cost, and the HEMT may have a broader application range.